1. Field of the Invention
This invention relates to a process and apparatus for operating loudspeakers, and more particularly, to a process and apparatus for operating loudspeakers in response to a high frequency signal superimposed on a low frequency sound signal to minimize sound distortion.
2. Description of the Prior Art
With the exception of an ion loudspeaker, which operates without a diaphragm, most loudspeaker systems using a diaphragm are afflicted with common fundamental deficiencies. In such loudspeaker systems, the quality of sound transmission is substantially determined by the quality of a loudspeaker diaphragm and to a lesser extent by associated loudspeaker electronic circuits. The usual loudspeaker system includes a fixed diaphragm for coupling sound vibrations to air, and a driver system responsive to an electrical current or voltage signal for producing mechanical forces which vibrate the diaphragm. The mechanical forces are substantially proportional to the magnitude of the current or voltage signal. Since the diaphragm has a mass which vibrates or changes position in proportion to the magnitude of the driver system output signals, the mean or average position of the diaphragm is maintained by operating the loudspeaker as a linear oscillator having one or more natural resonant frequencies. As a consequence, a diaphragm characteristic transmission curve of sound pressure with respect to frequency is determined by one or more resonance curves. These curves are additionally influenced by diaphragm radiation resistance or the coefficient between radiation and efficiency of a diaphragm having finite dimensions.
In typical loudspeaker systems, the total auditory frequency range is usually divided into two or more predetermined frequency ranges in order to obtain an approximately linear sound pressure curve over the entire auditory range. Signals within these frequency ranges are coupled to especially designed loudspeaker systems having diaphragms adapted to efficiently transmit acoustic signals within predetermined frequency bandwidths.
Prior art loudspeaker systems are inherently narrow band or have a limited range of frequency response. For example, it is difficult to reproduce relatively low (1 Hz) or relatively high (20 KHz) frequencies. In addition to having a relatively poor frequency response, this narrow band characteristic of prior art loudspeakers also produces phase distortion and erroneous pulse and transient response. A prior art solution to this problem has been to increase self-damping of the diaphragm. However, an increase in self-damping often results in a decrease in loudspeaker efficiency. The decrease in loudspeaker efficiency can be offset by modern transistor amplifiers adapted to amplify relatively low power signals to provide the desired power at the output of a loudspeaker system.
Modern loudspeakers, especially systems for middle and high frequency ranges, are so thoroughly damped that diaphragm motion is substantially determined by active mechanical or viscous elastic damping and less by blind or mass damping. Viscous elastic damping is achieved by impregnating a bead portion or radiation surface of the diaphragm with relatively tough elastic polymers. Such materials not only provide a desired friction drag proportional to velocity but also a diaphragm spring characteristic which depends on the diaphragm velocity. As a consequence, the diaphragm spring deviates from an ideal expansion and contraction along a straight line in response to an electric field and current signal. The diaphragm spring typically splits into two direction dependent branches resulting in a hysteresis loop which produces undesirable nonlinear distortions of sound.
A similar sound distortion problem occurs with glide centered loudspeakers which are guided by means of a rigid slide bearing instead of by means of a centering spider such as used in a cone or taper loudspeaker. A rigid slide bearing is advantageously used with bass loudspeakers since they permit relatively small loudspeaker systems to radiate sound signals with relatively large amplitudes. However, problems entailed in the use of a rigid slide bearing in a glide centered loudspeaker involving a difference between adhesive and sliding friction producing nonlinear distortions of sound and mechanical hysteresis have not hitherto been solved.